Refraction of light in a pool

1. The problem statement, all variables and given/known data
At the shallow end of a swimming pool, the water is 70.0 cm deep. The diameter of the cone of the light emerging from the water into the air above, emitted by a light source 10.0 cm in diameter at the bottom of the pool and measured by an observer on the edge of the pool 2.50 m away, is
a) 1.60 m
b) 1.70 m
c) 1.75 m
d) 1.80 m

Staff: Mentor

This is a critical angle problem. Draw a diagram of the light (at the bottom of the pool) being emitted at all angles. Figure out the maximum angle that will be refracted--anything greater will just be reflected by the water surface--and use that to calculate the diameter of the cone.